cmd/6c, cmd/8c: cut stack frames by about half

The routine that adds an automatic to the stack was
adding ptrsize-1 to the size before rounding up.
That addition would only make sense to turn a round down
into a round up. Before a round up, it just wastes a word.

The effect was that a 6c function with one local and
one two-word function call used (8+8)+(16+8) = 40 bytes
instead of 8+16 = 24 bytes.

The wasted space mostly didn't matter, but one place where
it does matter is when trying to stay within the 128-byte
total frame constraint for #pragma textflag 7 functions.

This only affects the C compilers, not the Go compilers.

5c already had correct code, which is now copied to 6c and 8c.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7303099
cmd/6c: fix build

copy+paste error while cleaning up CL 7303099 before submit

R=ken2
CC=golang-dev
https://codereview.appspot.com/7308104